posthog-security-basics

PostHog Security Basics

Overview

Secure PostHog API key management, least-privilege access, and secret rotation. PostHog has two key types with very different security profiles: the Project API Key (phc_...) is intentionally public and safe to include in frontend bundles, while the Personal API Key (phx_...) grants admin access and must never be exposed.

Prerequisites

• PostHog account with admin access
• Understanding of environment variable management
• .gitignore configured

Instructions

Step 1: Understand Key Security Profiles

Key Type	Prefix	Exposure Risk	Capabilities
Project API Key	phc_	Low (designed to be public)	Capture events, evaluate flags, identify users
Personal API Key	phx_	Critical (full admin access)	CRUD flags, read persons, query insights, delete data

# .env (NEVER commit)
NEXT_PUBLIC_POSTHOG_KEY=phc_abc123   # Safe for frontend (NEXT_PUBLIC_ prefix)
POSTHOG_PERSONAL_API_KEY=phx_xyz789  # Server-only — NEVER in frontend code
POSTHOG_PROJECT_ID=12345

# .gitignore
.env
.env.local
.env.*.local

Step 2: Create Scoped Personal API Keys

set -euo pipefail
# Create a read-only key for BI dashboards
curl -X POST "https://app.posthog.com/api/personal_api_keys/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "label": "bi-dashboard-readonly",
    "scopes": ["insight:read", "dashboard:read", "query:read"]
  }'

# Create a key scoped to feature flags only
curl -X POST "https://app.posthog.com/api/personal_api_keys/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "label": "feature-flag-service",
    "scopes": ["feature_flag:read", "feature_flag:write"]
  }'

Step 3: Rotate Personal API Keys

set -euo pipefail
# 1. Create new key in PostHog Settings > Personal API Keys
# 2. Update secret in your deployment platform
# Vercel:
vercel env rm POSTHOG_PERSONAL_API_KEY production
vercel env add POSTHOG_PERSONAL_API_KEY production

# GitHub Actions:
gh secret set POSTHOG_PERSONAL_API_KEY --body "phx_new_key_here"

# 3. Verify new key works
curl -s "https://app.posthog.com/api/projects/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" | jq '.[0].name'

# 4. Delete old key in PostHog dashboard

Step 4: Prevent Key Leaks

# Git pre-commit hook to catch PostHog personal keys
# .git/hooks/pre-commit (or use husky)
#!/bin/bash
if git diff --cached --diff-filter=ACM | grep -qE 'phx_[a-zA-Z0-9]{20,}'; then
  echo "ERROR: PostHog personal API key (phx_) detected in staged files!"
  echo "Remove it and use environment variables instead."
  exit 1
fi

# .github/secret-scanning.yml (or use GitHub's built-in secret scanning)
patterns:
  - name: PostHog Personal API Key
    regex: 'phx_[a-zA-Z0-9]{20,}'
    severity: critical

Step 5: Server-Side Key Isolation

// lib/posthog-server.ts — Personal key never leaves the server
import { PostHog } from 'posthog-node';

const posthog = new PostHog(process.env.NEXT_PUBLIC_POSTHOG_KEY!, {
  host: 'https://us.i.posthog.com',
  // personalApiKey ONLY used server-side for local flag evaluation
  personalApiKey: process.env.POSTHOG_PERSONAL_API_KEY,
});

// API routes that proxy admin operations
// Never expose the personal key to the client
export async function getFeatureFlagsForUser(userId: string) {
  return posthog.getAllFlags(userId);
}

Step 6: Audit API Key Usage

set -euo pipefail
# Check activity log for API key operations
curl "https://app.posthog.com/api/projects/$POSTHOG_PROJECT_ID/activity_log/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" | \
  jq '[.results[] | select(.scope == "PersonalAPIKey") | {
    user: .user.email,
    activity: .activity,
    created_at
  }]'

Security Checklist

• Project key (phc_) used for all frontend/capture code
• Personal key (phx_) only on server, never in frontend bundles
• .env files in .gitignore
• Separate keys per environment (dev/staging/prod)
• Scoped personal keys (not full admin for every service)
• Git pre-commit hook scanning for phx_ keys
• Key rotation documented and scheduled (quarterly)

Error Handling

Issue	Detection	Fix
Personal key in git history	GitHub secret scanning alert	Rotate key immediately, revoke old one
Wrong key type	401 on admin API	Use phx_ for admin, phc_ for capture
Overprivileged key	Audit log shows unexpected operations	Create scoped key, revoke broad one
Key exposed in logs	Log scrubbing finds phx_	Redact logs, rotate key

Output

• Environment-specific API key configuration
• Scoped personal API keys for least privilege
• Key rotation procedure
• Git pre-commit hook for leak prevention
• Audit log queries for key usage monitoring

Resources

• PostHog API Overview
• PostHog Privacy & Security

Next Steps

For production deployment, see posthog-prod-checklist.